Refrigeration systems for attaining temperatures down to about -40.degree. C., such as those used in domestic refrigerators and freezers, operate on the vapor-compression cycle. In the first step of this cycle, a low-pressure refrigerant vapor is compressed by a simple oil-lubricated compressor, such as a rotary vane or piston compressor. The warm compressed vapor then enters an air-cooled condenser where it loses heat and condenses. The condensed liquid refrigerant, with some entrained oil dissolved in it, passes through a fine capillary tube, throttle, or restriction into a larger chamber at a lower pressure, where it evaporates and absorbs heat. The low-pressure refrigerant vapor and the oil are then returned to the intake of the compressor, closing the cycle.
Lower temperatures, down to the -50.degree. C. to -100.degree. C. range, can be achieved by cascading two vapor-compression cycle refrigeration systems. The first system refrigerates down to the -20.degree. C. to -40.degree. C. range as described above, while the second system refrigerates further down to the -50.degree. C. to -100.degree. C. range using a low-boiling-point refrigerant. In order to operate such is cascade refrigerators continuously, the concentration of oil in the low-boiling-point refrigerant must be kept low enough so that it remains dissolved in solution and does not clog the low-temperature capillary, blocking the refrigerant flow. Although it is possible to attain temperatures below -100.degree. C. using these techniques, such cascaded refrigeration systems have clogging problems at these lower temperatures. Moreover, one must either use additional refrigeration circuits or higher pressure compressors, both of which add cost and complexity to the refrigerator.
Temperatures down into the -100.degree. C. to -200.degree. C. range also have been achieved using a single refrigerant stream with a mixture of several refrigerants having different boiling points. In this method of refrigeration, a simple compressor pressurizes the refrigerant mixture, some portion of which condenses when cooled to ambient temperature by an air-cooled condenser. The liquid portion is then separated from the vapor portion in a liquid-vapor separator and allowed to expand through a capillary, causing it to evaporate and cool. The evaporated liquid passes through a heat-exchanger where it cools the vapor coming from the separator, and then flows back to the compressor. Meanwhile, as the vapor coming from the separator is cooled in the heat-exchanger, a portion of it condenses. This condensed portion is then separated from the remaining vapor portion, evaporated, and used to cool the remaining vapor portion further, just as before. Several such stages of liquid-vapor separation and counter-current heat exchange are used to reach the lowest refrigeration temperature.
In this type of refrigeration system, oil from the compressor is largely concentrated in the liquid fraction of the first liquid-vapor separator and returned to the compressor via the first counter-current heat exchanger. Likewise, the higher-boiling-point components of the mixture are successively removed from the refrigerant stream as it proceeds through the stages to the lowest temperature stage, thus removing these components from the stream before they can freeze in the lower temperature capillaries and clog the system. This refrigeration method, however, does not provide an effective means for purging the refrigerant stream of all high-molecular-weight contaminants that can clog the flow at low temperatures. Moreover, the phase separators add cost and complexity to the refrigeration system.
The principles of these single-stream mixed-refrigerant systems were first described by A. P. Kleemenko, "One Flow Cascade Cycle", Proceedings of the Xth International Congress on Refrigeration, Copenhagen, 1, 34-39, (1959), Pergamon Press, London. They have subsequently been described in texts of cryogenic refrigeration systems, such as "Theory and Design of Cryogenic Systems" by A. Arkjarov, I. Marfenina and Ye. Mikulin, Mir Publishers, Moscow (1981). An important improvement in the cycle was described by D. J. Missimer in U.S. Pat. No. 3,768,273 issued in 1973. Missimer obtained more stable and lower pressure operation by making only a partial liquid-vapor separation at each stage rather than a complete separation. Nevertheless, Missimer's improvement on this type of refrigeration system still has low temperature clogging problems due to high-molecular-weight contaminants in the refrigerant stream, and still requires the use of several phase separators.
Other authors have described the use of mixed-gas refrigerants to attain low temperatures without the use of expensive phase separators. Most notable are those refrigerant mixtures containing a mixture of nitrogen with some of the lighter hydrocarbon gases, such as methane, ethane, propane, and iso-butane. Similar mixtures containing, in addition, some of the Freons have been described by Alfeev, Brodyansky, Yagodin, Nikolsky & Ivantsov, British Patent 1,336,892 (1973); W. A. Little, Proceedings of the 5th Cryocooler Conference, Monterey, (1988); W. A. Little, Advances in Cryogenic Engineering, 1305-1314 (1990); C. K. Chan, Proceedings of Interagency Cryocooler Meeting on Cryocoolers, p. 121 (1988), and R. Longsworth, U.S. Pat. No. 5,337,572 (1994).
As Chan and Little have noted, although refrigeration system using these refrigerant mixtures can attain low temperatures without using phase separators, experience has shown that prolonged refrigeration at these temperatures can only be achieved if the gas stream is cleansed of condensable contaminants. Present methods for cleaning the working fluid of oil residues, contaminants, and water vapor involve introducing filters (e.g., a molecular sieve or a series of activated charcoal adsorption filters) into the high-pressure line or pressure-swing dual-adsorption columns. These filters, however, are expensive and add complexity to the system. Moreover, they add substantially to the volume of the refrigeration system, resulting in refrigeration systems that are large, bulky, and have start-up problems.
Contaminants that can cause clogging of the capillaries or expansion valves are of two general classes. The first class of contaminants includes the residual oil that remains in the stream after it passes through the oil separator. This residual oil can precipitate out of the refrigerant solution at the lowest temperatures and cause clogging. The second class of contaminants includes the products from reactions between the oil and the refrigerants, as well as high-molecular-weight residues extracted over time from various sources in the compressor such as the wire insulation, the lubricant used for winding the wire, plastic insulation, castings, the oil, and the case of the compressor. Although a simple cyclone oil separator can remove much of the entrained oil from the hot vapor coming from the compressor, it is ineffective in removing the more complex residues in the second class of contaminants.